Solid State Phenomena Vols. 217-218

Abstract: We have developed new type semi-solid injection process for magnesium alloy. This process does not require to use any cover gases and the special magnesium billet such as thixo-billet. In this study, plate specimens were produced by injecting the semi-solid billet with different fraction solid. The microstructure observation, detection of casting defects by an X-ray computed tomography scanner, and tensile test were carried out. With increasing fraction solid, the size and shape of α-Mg solid particles became smaller and more spherical. In the condition of low fraction solid or forming in liquid state, the casting defects were located in the center of the specimen at the thickness direction. Additionally, the volume fraction of the casting defect decreased with increasing fraction solid. Moreover, the casting defects can be reduced by preventing solidifying and clogging of the top of the nozzle. Then, the specimen which has few casting defects could be obtained by injecting the slurry of fraction solid 0.5. However, the tensile strength and yield strength were highest in fraction solid 0.4. It is contemplated that the composition of the solid solution component element in the matrix was increased in fraction solid of 50%, therefore the matrix became brittle.

Abstract: In this paper evolution from hot forging to thixoforging process is proposed and talked over. A real-case study is presented, namely a steel-made steering piston produced by thixoforging technique is considered and some parameter has been numerically analyzed. The transition between the traditional forging process and the new one involves several transformations and presents difficulties mainly due to the high working temperature. The targeted thixoforging technology allows obtaining near-net-shape components with a significant reduction as the starting material quantity concerns, and implicitly of the involved amount of energy.

Abstract: Investigations on steel thixoforming started in the early 90s and many developments were carried out, mainly driven by the industrialization of the process. After a series of basic investigations on adapted steels and materials as well as near-net shaping investigations, the issue of mass production raised and thus induced some other challenges mainly on die life and process automation and cycling. High working temperatures and components complexity were than the main locks to mass productions. In most of reported literature, none of the involved research group reached or real production and industrialization criteria. The euro group holds most of these research investigations. We have reported many of these results in previous communications and this paper presents a continuity of what have already been presented. Some advanced tooling materials and solutions are presented and applied to a real complex industrial component.

Abstract: A combination of hot rolling and equal channel angular pressing (ECAP) was explored to generate globular microstructures in the Mg-3%Zn alloy after re-heating to the semisolid state. It was found that the single-step deformation of as-cast alloy via hot rolling at 350°C with a thickness reduction of 50% refined the alloy microstructure by creating deformation bands of the Mg (α) phase with a size of the order of tenths of micrometers. After re-heating to 630 °C, the microstructure transformed into spheroidal morphologies with an average globule size of 82 μm. An additional deformation of the hot-rolled alloy by the ECAP method at 250 °C further refined the alloy microstructure to sub-micrometer grains of lath and equiaxed shapes. After re-heating of this microstructure to 630 °C the average globule size reached 62 μm, which is roughly 25% smaller than that achieved for the hot-rolled precursor. The role of strain-induced melt activation (SIMA) techniques in generation of globular morphologies in Mg-based alloys after partial re-melting is discussed.

Abstract: Al-Mg₂Si composites have gained considerable attention because of their attractive properties such as low density, improved wear resistance and good castability. However, when in-situ routes are used for processing of these composites, the formation of coarse Mg₂Si particles with sharp cornersis inevitable and is detrimental for the composite properties. This problem is intensified when a hypereutectic composition such as Al-25wt.% Mg₂Si alloy is processed. In the present study, an innovative semisolid technique termed as the Vibrating Cooling Slope (VCS) has been applied to produce a sample of in-situ Al-25wt.% Mg₂Si composite.This technique combines the conventional cooling slope and vibration casting methods into an integrated one for producing fine and globular structures in the as-cast condition. An inclined plate was prepared from a 10mm thickness copper plate and was coated by boron nitride. This platecould vibrate mechanically in the vertical direction at a predetermined frequency by the aid of four springs and an electric motor. The molten Al-16.5wt.%Mg-9.4%Sialloy with 100^o C superheat was poured on the surface of this cooling slope (set at 45° inclined angle)while it was vibrating at the frequency of 40 Hzandamplitude of 400 μm.The semisolid alloy travelled the length of 40 cm on the slope before being poured into a steel mold (60 mm internal diameter and 35 mm in height). Also for the purpose of comparison, gravity casting (GC) and conventionally still cooling slope casting (CS)were carried outboth using the same mold and with the same superheat.The samples were sectioned, polished and subjected to metallographicstudies,porosity and hardness measurements. It was concluded that CS and VCS techniques resulted in a decrease in the size of Mg₂Si particles by about 50%and 70% respectively when compared with the gravity casting. However, the increased shape factor of Mg₂Si particles in the VCS and CS processed samples was insignificant as compared with GC. Although in comparison with GC, the VCS processed sample showed a higher porosity level, it exhibited a higher hardness value. These results were attributed to the finer and modified microstructure obtained via this newly developed technique.

Abstract: Al-Si-Fe alloys are a general purpose of die casting alloys, widely used to manufacture automotive parts. Forming this alloy in semi-solid state can eliminate important problems in die casting process. In the present work, low superheat melt was employed to produce modified microstructure and non-dendritic A380 alloy feed stocks. The melt was cast on a cooling slope plate at 615 °C to obtain ingots. Then, ingots are thixoformed using a hydraulic press after heating them at 570 °C for 15 minutes, yielding a microstructure with predominantly α–Al globules, Si particles, and modified β-phase intermetallic compounds. The effect of semi-solid processing on the morphology, size, and distribution of iron-bearing intermetallics was studied. The results of image analysis of the samples showed that by using semi-solid method, the shape factor of α-Al phase improved, and iron containing intermetallics were modified in size and distribution. These changes in the aspect ratio and redistribution of the intermetallics improved the mechanical properties such as hardness of the products.

Abstract: The present study has been taken up to establish fracture property correlation of the semi solid processed 6061 aluminium alloy. Electro Magnetic Stirring (EMS) has been performed to get non-dendritic billets and then the alloy has been thixocast using a High pressure die casting machine. Tensile tests have been performed to estimate mechanical properties of the semi solid processed alloy. Fracture behaviour of the alloy has been investigated at the intermediate EMS cast state and subsequent final thixocast state, employing uniaxial tensile tests at 0.001s^-1 strain rate and at room temperature. Fracture surface morphology of the EMS cast state shows that the presence of micro porosity is responsible for crack initiation and final fracture. Our results confirm that the dimpled rupture of primary Al phase is responsible for improved mechanical properties in the thixocast state of the alloy.

Abstract: An experimental equipment with inclined plate cooling and shearing had been developed to prepare semi-solid 9Cr18 martensitic stainless steel slurry. The effects of inclined plate angel and length on microstructures refinement and spheroidization in semi-solid 9Cr18 alloy was investigated. The results show that when the molten alloy is flowing along the plate surface, the microstructure of the alloy involves from coarse dendrite to fine spherical grains, the sloping plate can provide sufficiently the undercooling for nucleation and lay a foundation for heterogeneous nucleation to form lots of nuclei of crystal internally and on the sloping plate surface, thus causing the melt to nucleate eruptively. Sloping plate angle and sloping plate length can affect the shear strength and shear time of the flow alloy on the sloping plate and thus influence alloy microstructure. Fine spherical grain in the semi-solid 9Cr18 slurry can be obtained at α=45°and L=600mm, the major grain equivalent diameters are less than 55μm and more than 40% of the grain shape factor is in 0.75, exhibiting better grain fine degree and roundness. Using energy dispersive spectrometer (EDS) to investigate the liquid and solid phase in the 9Cr18 semi-solid slurry, iron content is lower and carbon and chromium content is higher in the liquid phase than in the solid phase which makes it possible to prepare the FGM (functional gradient material) with a high wear resistance in the surface and a good strength and toughness in the center.

Abstract: Semi-solid billet of ZCuSn10 (Wt%: 88.25Cu, 10.48Sn) alloy is prepared by strain induced melt activated (SIMA) method which including rolling and remelting process. Firstly, ZCuSn10 alloy is casted, and rolling samples are cut from ingot casting. Secondly, the rolling samples are two pass or four pass rolled after holding 15 minutes at 450°C, then samples with 10% and 20% pre-deformation degree are obtained. The remelting samples are cut from pre-deformed samples. Lastly, the remelting samples are reheated up to 850°C or 875°C, water quenching after holding for 15 minutes. Then semi-solid microstructure of ZCuSn10 alloy is prepared. The semi-solid microstructure of ZCuSn10 alloy is observed and compared with annealed microstructure and microstructure of ZCuSn10 alloy directly remelted after casting. The results indicate that semi-solid microstructure of ZCuSn10 alloy by rolling-remelting SIMA process is uniform and fine grain, and spheroidization level of solid particle is well. The optimum semi-solid microstructure is obtained when alloy with pre-deformation 20% is remelted at 875°C for 15 minutes, the average grain diameter is about 75.80μm, shape factor is 1.62, and volume fraction of liquid phase reaches about 17.28%. Pre-deformation process plays a crucial role in grain refinement and spheroidization during SIMA process for preparing the semi-solid ZCuSn10 alloy, as pre-deformation degree and remelting temperature increases, volume fraction of liquid phase increases, the solid particles in semi-solid microstructure are smaller and rounder. The main mechanism of SIMA process preparing semi-solid billet of ZCuSn10 alloy is that pre-deformation breaks dendrites and stores energy of deformation into alloy, and promotes dendrites fusing through remelting process. Meanwhile, liquid phase occupies sharp corners of solid particles by Sn element diffusing from liquid phase into α solid phase, so that fine and uniform and globular α solid particles are gained.

Abstract: The production of nanoreinforced aluminum alloys in volume and quality suitable for subsequent shape casting has been problematic. Large specific surface area and high interfacial energy of the particles combined with high surface tension of the aluminum melt makes it difficult to add appreciable numbers of particles to the melt, even when later de-agglomerated by techniques such as ultrasonic cavitation. Previous work by the authors used semi-solid squeeze casting to produce master alloys for dilution into the casting alloy. While the technique was shown to be effective, the master alloy proved difficult to remelt and was expensive to produce. The objective of this new work was to use an extrusion process that could be more readily scaled to produce nanoreinforced aluminum master alloys. This paper describes the process developed for incorporation of nanoparticles using semi-solid extrusion and the results of alloys produced using that process.